Vesicular trafficking is a process that spans from the cellular membrane to the cell’s organelles. Cells interact with their surroundings by secreting or importing proteins. These proteins are packaged in vesicles. At the membrane, endocytosis and exocytosis permit communication with the environment. These actions also ensure the renewal of receptors, ion channels, surface proteins, and membrane lipids. Inside the cell, an organized trafficking system ensures vesicles are directed to the correct place. In the cytoplasm, vesicles travel along microtubules. They use motor proteins to shuttle to different organelles.

This is one of the most detailed pathways for endocytosis, which allows the cell to internalize substances. The process begins when a ligand binds to its receptor on the cell surface. A signal is then sent inside the cell to recruit clathrin proteins. Clathrin proteins are arranged in a triskelion shape. These triskelions bind to adaptor proteins on the forming vesicle. They function by mechanically deforming the membrane. This action forms a rounded vesicle. The membrane invaginates into a small coated pit, which then grows into a coated vesicle. A GTPase protein called dynamin ensures the vesicle separates from the membrane. Dynamin forms a helix around the vesicle’s neck to pinch it off. After separation, the clathrin proteins disassemble from the vesicle. The vesicle is then shuttled to the early endosome.

Exocytosis is a process that allows for the secretion of proteins, hormones, and neurotransmitters. It is also involved in antigen presentation and the turnover of plasma membrane components. The process involves four steps: approach, tethering, docking, and fusion. First, vesicles approach the plasma membrane, using motor proteins like kinesin or dynein that move on a microtubule track. Second, the vesicle loosely attaches to the membrane in a step called tethering. This occurs when Rab-GTP on the vesicle binds to a Rab effector protein at the target membrane. Third and fourth, vesicle docking and fusion are managed by SNARE proteins. There is a v-SNARE (vesicle SNARE) and t-SNAREs (target SNAREs). The binding of the v-SNARE to the t-SNAREs causes the fusion of the vesicle to the plasma membrane.

Vesicles are actively transported within the cytoplasm by motor proteins. These proteins are described as molecular motors. They use microtubules as intracellular tracks to move along. Motor proteins provide a mechanical force to ensure vesicle transport, a process that is fueled by ATP hydrolysis. The two main types of these proteins are kinesin and dynein. Kinesin proteins move from the minus end of the microtubules toward the plus end. This protein’s function was studied in neurons, where it transports synaptic vesicles along the axon. Dynein transports vesicles in the opposite direction, from the plus end toward the minus end. This direction of movement is necessary for retrograde transport in neurons.